To perform position location in wireless cellular networks (e.g., a cellular telephone network), several approaches perform trilateration based upon the use of timing information sent between each of several base stations and a mobile device, such as a cellular telephone. One approach, called Advanced Forward Link Trilateration (AFLT) in CDMA or Enhanced Observed Time Difference (EOTD) in GSM or Observed Time Difference of Arrival (OTDOA) in WCDMA, measures at the mobile device the relative times of arrival of signals transmitted from each of several base stations. These times are transmitted to a Location Server (e.g., a Position Determination Entity (PDE) in CDMA), which computes the position of the mobile device using these times of reception. The transmit times at these base stations are coordinated such that at a particular instance of time, the times-of-day associated with multiple base stations are within a specified error bound. The accurate positions of the base stations and the times of reception are used to determine the position of the mobile device.
FIG. 1 shows an example of an AFLT system where the times of reception (TR1, TR2 and TR3) of signals from cellular base stations 101, 103 and 105 are measured at the mobile cellular telephone 111. This timing data may then be used to compute the position of the mobile device. Such computation may be done at the mobile device itself or at a location server if the timing information so obtained by the mobile device is transmitted to the location server via a communication link. Typically, the times of receptions are communicated to a location server 115 through one of the cellular base stations (e.g., base station 101, 103 or 105). The location server 115 is coupled to receive data from the base stations through the mobile switching center 113. The location server 115 may include a base station almanac (BSA) server, which provides the location of the base stations and/or the coverage area of base stations. Alternatively, the location server 115 and the BSA server may be separate from each other; and the location server 115 communicates with the base station to obtain the base station almanac for position determination. The mobile switching center 113 provides signals (e.g., voice communications) to and from the land-line Public Switched Telephone Network (PSTN) 117 so that signals may be conveyed to and from the mobile telephone to other telephones (e.g., land-line phones on the PSTN 117 or other mobile telephones). In some cases the location server 115 may also communicate with the mobile switching center 113 via a cellular link. The location server 115 may also monitor emissions from several of the base stations 101, 103, 105 in an effort to determine the relative timing of these emissions.
In another approach, called Uplink Time of Arrival (UTOA), the times of reception of a signal from a mobile device is measured at several base stations (e.g., measurements taken at base stations 101, 103 and 105). FIG. 1 applies to this case if the arrows of TR1, TR2 and TR3 are reversed. This timing data may then be communicated to the location server 115 to compute the position of the mobile device.
Yet a third method of doing position location involves the use in the mobile device of circuitry for the United States Global Positioning Satellite (GPS) system or other Satellite Positioning Systems (SPS), such as the Russian GLONASS system and the proposed European Galileo System or a combination of satellites and pseudolites. Pseudolites are ground-based transmitters, which broadcast a PN code (similar to a GPS signal) modulated on an L-band carrier signal, generally synchronized with SPS time. Each transmitter may be assigned a unique PN code so as to permit identification by a mobile device. Pseudolites are useful in situations where SPS signals from an orbiting satellite might be unavailable, such as tunnels, mines, buildings or other enclosed areas. The term “satellite”, as used herein, is intended to include pseudolite or equivalents of pseudolites, and the term GPS signals, as used herein, is intended to include GPS-like signals from pseudolites or equivalents of pseudolites. Methods that use an SPS receiver to determine a position of a mobile station may be completely autonomous (in which the SPS receiver, without any assistance, determines the position of the mobile station) or may utilize the wireless network to provide assistance data or to share in the position calculation. Examples of such methods are described in U.S. Pat. Nos. 5,812,087; 5,841,396; 5,874,914; 5,945,944 and 6,208,290. For instance, these patents describe, among other things: a method to obtain from cellular phone transmission signals accurate time information, which is used in combination with SPS signals to determine the position of the receiver; a method to transmit the Doppler frequency shifts of in-view satellites to the receiver on the mobile device through a communication link to determine the position of the mobile device; a method to transmit satellite almanac data (or ephemeris data) to a receiver through a communication link to help the receiver to determine its position; a method to lock to a precision carrier frequency signal of a cellular telephone system to provide a reference signal at the receiver for SPS signal acquisition; a method to use an approximate location of a receiver to determine an approximate Doppler for reducing SPS signal processing time; and a method to compare different records of a satellite data message received to determine a time at which one of the records is received at a receiver in order to determine the position of the receiver. In practical low-cost implementations, both the mobile cellular communications receiver and the SPS receiver are integrated into the same enclosure and may in fact share common electronic circuitry.
In yet another variation of the above methods, the round trip delay (RTD) is found for signals that are sent from the base station to the mobile device and then are returned. In a similar, but alternative, method the round trip delay is found for signals that are sent from the mobile device to the base station and then returned. Each of these round-trip delays is divided by two to determine an estimate of the one-way propagation delay. Knowledge of the location of the base station, plus a one-way delay constrains the location of the mobile device to a circle on the earth. Two such measurements from distinct base stations then result in the intersection of two circles, which in turn constrains the location to two points on the earth. A third measurement (even an angle of arrival or cell sector identification) resolves the ambiguity.
A combination of either the AFLT or U-TDOA with an SPS system may be referred to as a “hybrid” system. For example, U.S. Pat. No. 5,999,124 describes, among other things, a hybrid system, in which the position of a cell based transceiver is determined from a combination of at least: i) a time measurement that represents a time of travel of a message in the cell based communication signals between the cell based transceiver and a communication system; and ii) a time measurement that represents a time of travel of an SPS signal.
Altitude aiding has been used in various methods for determining the position of a mobile device. Altitude aiding is typically based on a pseudo-measurement of the altitude. The knowledge of the altitude of a location of a mobile device constrains the possible positions of the mobile device to a surface of a sphere (or an ellipsoid) with its center located at the center of the earth. This knowledge may be used to reduce the number of independent measurements required to determine the position of the mobile device. For example, U.S. Pat. No. 6,061,018 describes, among other things, a method where an estimated altitude is determined from the information of a cell object, which may be a cell site that has a cell site transmitter in communication with the mobile device.